Description of the Prior Art
U.S. Pat. No. 3,395,603 to E. J. Skierski discloses a drill screw of the type over which the present invention provides improvements. The Skierski drill screw comprises an elongated element having a driving head at one end, a threaded central region and a drill bit formed at the opposite end. The drill bit is formed with flutes extending therealong in diagonally opposed quadrants with one surface of each flute cooperating with the peripheral surface of the drill bit and tapered point portions of the drill bit to form cutting edges. The outer periphery of the shank portion of the drill bit is elliptical in cross-section and the surfaces of the flutes which define the cutting edges intersect the outer periphery of the drill bit shank portion at the major axis of the ellipse. The purpose for this is to provide cutting edges at locations farthest away from the central axis of the screw and to provide a peripheral relief behind each cutting edge.
The above identified Skierski patent also shows a pair of point forming dies which close upon each other and pinch the end of a screw blank between them. The dies have identations on their mutually facing working surfaces which conform to the shape of the drill bit to be formed on the end of the screw. When the dies close on each other they pinch the metal of the screw blank and cause it to flow into and conform to the shape of the identations. The Skierski patent does not describe how the die identations are formed; but in any event curved surfaces of the die must describe elliptical segments so that the drill bit which they provide will have the elliptical cross-section required by the Skierski patent.
There are certain disadvantages which result from the elliptical cross-section employed by Skierski to obtain peripheral relief behind each drill bit cutting edge. Firstly, the plane of the outer periphery of the drill bit is always perpendicular to the direction of movement of the cutting edge so that relief is not provided immediately behind the cutting edge. Secondly, because the plane of the outer periphery of the drill bit is perpendicular to the direction of movement of the cutting edge, the angle between the peripheral plane and the intersecting flute plane along the cutting edge is never less than 90.degree. so that the sharpness of the cutting edge is limited. Finally, the elliptical cross-section described by Skierski is quite difficult to obtain in a mass production operation. This is because the dies which are used to form the screw drill points must be indented to the elliptical cross-sectional shape, and the techniques for accomplishing this, i.e., electrical discharge machining and hobbing, are difficult and expensive. Electrical discharge machining is not adaptable to the high production rates needed for commercial die manufacture. Although hobbing is adaptable to commercial production, it is very difficult, time-consuming and expensive to grind hobs to an elliptical contour because the grinding wheel used to shape the hob must itself be dressed to an elliptical contour; and the dressing stone which shapes the grinding wheel must be moved synchronously about two axes during the dressing operation.